Moteino USB - Reduce FTDI current on sleep

[TomWS]

Actually -- very good point about the diode reverse current. I was using this diode  (SD0805S040S0R5), but it appears to have 0.1mA as leakage!...

No wonder it's shown as a resistor!

BTW, are you really using 47 ohm resistors for the LED current limits?

[akstudios]

No wonder it's shown as a resistor!

BTW, are you really using 47 ohm resistors for the LED current limits?

Hah! Good one.

Nah, I'm using 1k ohms on the actual board. Reused resistor symbols from some of my older projects and forgot to change the value in schematic. I mean, it shines brighter if you do use 47 ohm..

[akstudios]

Well done!

I noted earlier that the FT231X has 200k pull-ups on the inputs on the UART signals to VCCIO. Maybe the combination of that current plus the leakage through the diode was enough to parasitically power the chip, but remove one of them (i.e. use a low leakage diode, or stop the RX pin leakage via the 200k resistor by isolation) and the problem goes away?

Mark.

Yes, this is definitely it. Initially I thought the internal pull-ups were limited to a different package, but both are functionally the same. Blocking current from both sides makes it goes away.

[perky]

My guess is that the diode leakage allows VBUS (i.e. the VCC input of the FX231X) to rise. This is the input to a 3.3V regulator inside the FT231X, which could well have a clamping effect on it's 3.3V output and keep it below a threshold that enables some logic. Since VCCIO is also connected to that 3.3V output, any leakage from an input could cause it to rise *if* VCC is high enough. So, putting a low reverse current diode in, or blocking the leakage from the RX input, makes it work.

It would be interesting as to what happens both to the VCC input and the VCCIO during failure.

Mark.